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hairlets of hearing, which are subject to excitations of a very definite 
character from waves of sound. 
That even these instruments of the higher sensory perceptions 
in the lower worms—eyes and ears—are nothing else but specially 
developed parts of the outer skin-covering, is a fact of the greatest 
significance. For it is a fact also that the much more highly 
developed and elaborate eyes and ears of the higher animals, 
have arisen originally in the outermost skin-layer of the body, 
and do not contradict the highly important, and only recently 
firmly established law of the origin of all sensation from the skin. 
In respect of their origin, all the different instruments of sense in 
animals are only specialised parts of their sensory outer-skin. 
Furthermore, the instruments of motion, the servants of the 
will—the muscles, stand, in respect of their origin, in the closest 
connexion with the outer skin. 
In our lower worms, the whole muscle-system is represented 
by nothing more than a mere thin layer of flesh, which spreads out 
everywhere under the covering of skin. Usually this so-called 
‘‘skin-muscle-membrane” of worms separates into two different 
layers—an outer layer of round filaments, and an inner one of long 
filaments, but not yet separated into groups or bundles of muscles, 
as in the higher animals. 
But especial prominence must be given to the important fact 
that the whole body of werves, alike the centripetal sensory filaments 
that go from the brain to the skin and sense organs, and the © 
centrifugal motor filaments that go from the brain to the muscles, 
stand in immediate connexion with these outer-lying parts. Thus 
when, agreeably to nature, we conceive the whole soul-apparatus 
as one whole, the perceptive sensory organs are nothing else but 
special expansions of the ends of the sensory nerves; and the 
muscle filaments subservient to the will, are nothing else but special 
terminal organs of the motor nerves. Between those first and 
these last is intercalated the brain, as a common centre and direct 
bridge of connexion. 
If a clear perception is wanted of the working of such a soul- 
apparatus—of the nature of the soul-life, the best help for such a 
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